Let's say 12th level.
Human Champion Fighter:
107ish HPs
GWM and HWM feats (so cleave rider and -3 damage incoming per attack)
Tunnel Stalker for the melee control/OAs
18-19 AC (depending on magic items)
Human Totem Barbarian:
120ish HPs
GWM (so cleave rider and -3 damage incoming per attack) and Sentinel feats
Wolf Totem so allies have ADV on attacks against enemies w/in 5 of you
maybe 15 AC? (depending on magic items).
Just quoted the above build here.
The Barbarian IMC has AC 20 - +5 from CON, +2 from DEX, +3 for a +1 shield.
The Cleric has AC 21 - 19 from +1 plate, +2 for mundane shield.
The Fighter in the Primeval Thule game I play in has had AC 21 from 2nd level - 16 from bronze cuirass, +2 DEX, +2 shield, +1 defensive style. You get the same in a standard campaign with 18 from plate armour, +2 shield, +1 defensive.
Those are both low magic campaigns, in a standard magic campaign I'd expect 12th level PCs to have ACs in the low 20s.
Hrmmm...let me see. A 12th level Barbarian has 3 ASIs. If one feat (GWM) that becomes 2, if two feats (Sentinel), that becomes 1.
Human Totem Barbarian with Standard Array is:
15, 14, 16, 9, 13, 11. Assume you go 3 ASIs instead of feats. You put the 16 in Con. You put the 15 in Str and the 14 in Dex. ASI you go Con > Con > split Str/and the 13 (probably Wis).
Your Unarmored Defense AC is then 10 + 5 + 2 = 17. I guess if you aren't going GWF then you're going to want a shield. Shield +1 gets you to that AC 20.
You're HPs are another +2/level for 144ish with halved damage.
Weapon +2 nets you +4 (Prof) +3 (Str) + 2 (Ench) = +9 hit @ d8 +3 (Str) +3 (Rage) +2 (Enc) = 13.5 ave damage @ 80 % hit rate * 2 attacks.
This guy is beefy no doubt. However...
He can't go Reckless Attack against this number of enemies (return is terrible and the cost is significant). His offense is not great. There is a
fairly strong chance this guy is only killing one enemy per round.
He isn't sticky at all. He literally has no melee control. At 14th level, prone as Bonus Action comes online. Good stuff. But 0 control without feats.
The main deal here though is the
Wolf Totem Warrior effect. That would give the GWF Advantage on most attacks. That plus Bless will turn the Fighter into a terror.
I assumed he would be built as a 2H weapon berserker with control, but we can go with this I suppose.
Now the
Human Champion Fighter, assume something like:
16, 13, 15, 11, 9, 14. GWF + HAM + PAM + 2 ASI; +2 Str > +1 Str/Con. +1 Plate for 19 AC. +2 Halberd/Glaive.
With
Tunnel Stalker, GWF, PAM, +5 Str, +2 polearm, Advantage on most attacks (Wolf Totem Barb) and Bless, this guy is your melee control and meat-grinder. Also -3 damage per/attack due to HAM.
I'm still seeing 30 % and 35 % hit rates against these guys. And again, if the Barbarian goes to a build that is more effective in this scenario (IMO and especially so with the cleric in play here), he is going to be hit on a 10 - 12 (12 if the rare magic item becomes Bracers of Defense rather than +2 weapon).
Also, if they aren't keeping up with the waves (which seems very possible at some point with the above builds), the prospects for Help Actions (or Help + Grapple) by the Orcs goes up which could possibly turn into problem (that the cleric would have to bail them out of, assuming it isn't in trouble itself as the "reapers" get behind).
In that kind of a scenario, craploads is the right answer. I've never tested this, but I'm willing to put my head on the block and say over 100 waves easy provided the orcs don't get smarter and retreat. So between 100 waves and endless!
<snip>
And healed a LOT, don't forget the support class which will also be 12TH level. Regarding Bounded Accuracy, it mostly depends on the initiative order. That is key.
I think a fair bit of it depends on the control here. I don't like this Barbarian build above (no control and paltry offense...basically blocking terrain, a buff-bot, and maybe averaging taking out 1 of the 4 orcs). You put two warriors who can (a) be blocking terrain, (b) make the melee sticky and punish foes who violate their control, and (c) have the offensive output to keep up with the waves without being overrun, it becomes doable.
If a - c fall apart, the orcs overrun the town, or waylay (and possibly kill) the cleric (even though a War Cleric is so stout), become too many in number rendering the DPR input too significant to keep up with...or a combination of the above.
But I think with the right builds (sub defensive Barb for offensive one without Reckless Attack), a - c can be maintained and the Cleric can sustain the two reaping juggernauts for a long, long time.
If I can find some time, I'm going to impose my will on one of my players to run this with a War or Life Cleric.
Spitball, the orcs hit 25% of the time, and they do average 6.5 damage after the -3 DR, so each of the fighters will take 1 hit every other round (1 of the 4 orcs will hit one of them). That gives the fighter about 20 rounds, the barb a tiny more, plus whatever healing they can get. They'll definitely hold off on the order of low 100's of orcs. (I am just assuming no orc ever survives a round in combat here, which I think is a fairly safe assumption, though probably a tiny bit optimistic).
NOW, lets spitball a 1e AD&D fighter, level 10, 65 hit points, AC -4, damage is probably irrelevant (will slag an orc per attack automatically) Its getting 2 attacks/round per fighter, so things are about the same there, but the orcs only hit on a 20, which is a BIG difference. On average each fighter will take 15 hits to kill, roughly, and at 4 attacks per round, that will take 150 rounds. So your AD&D fighters can hold off 600 basic orcs. They're SOMEWHAT tougher than the 5e fighter/barb, but the difference isn't huge.
I still see the hit rate being higher than the 25 %. I think 30/35 or better (assuming an offensive Barbarian) is more realistic.
Also, consider the default sticky melee in AD&D.
Also, while the 1 attack/character level wouldn't apply to orcs, they would apply to most all canon fodder in AD&D.
Finally, consider the AD&D 2e Fighter. The Heroic Fray rules extend that to Level - 10 in HD so it would apply to orcs for level 12 Fighters. And the AD&D 2e weapon spec train had one destination; pain. I feel pretty confident that both the AD&D 1e and the AD&D 2e Fighter would soundly defeat the 5e analogue in this scenario.